Method and apparatus for homogenizing of bulk material

ABSTRACT

The invention relates to a method and an apparatus for homogenizing bulk material, wherein the bulk material is supplied from above into a container which is restricted sidewards and downwards and is fed out by means of a feeding out conveyor located in the lower portion of the container. The characteristic features of the invention are that the bulk material is fed in from above in one end of the container (20), wherein the bulk material is caused to slide down towards the opposite end of the container such that a bed of bulk material is formed having a sloping profile in the container, and wherein, due to the sliding, heavier and/or coarser fractions of the bulk material slides more than lighter fractions, and that during each time unit a volume of bulk material is fed out from the bottom portion of the container from each section (Δl) of the length of the container in the feeding out direction, said volume of bulk material corresponding to the supply of bulk material during the same time unit to the section (A n ) of the upper surface of the bed lying straight above said section of the length of the container, wherein the bulk material is caused to move essentially downwards in the bed along the entire length of the bed towards said outfeeder (30) which feeds out the material from each section at essentially the same rate as new material is supplied to the above lying surface section of the upper surface of the bed.

TECHNICAL FIELD

The invention relates to a method and an apparatus for homogenizing bulkmaterial, comprising a container having a rear end wall and a front endwall and two side walls, a feeding in conveyor for feeding in bulkmaterial from above into the container, which is provided to accommodatea bed of bulk material, the extension of said bed being limited by theend walls and the side walls; and a feeding out conveyor at the lowerpart of the container, said feeding out conveyor extending at leastbetween the end walls and being exposed against the bulk materialbetween the two end walls and provided to feed out the bulk material ina direction towards the front end wall.

BACKGROUND OF THE INVENTION

Bulk material containing solid particles usually has a varying particlesize distribution and/or mass distribution.

In the handling of such bulk material, which may consist e.g. of gravel,sand, asphalt or gravel mixtures, asphalt-concrete, moist concrete, orthe like, the bulk material usually is separated into fractionscontaining coarser and finer particles. When a container is being filledwith that type of bulk material by supply from above, e.g. from a point,the bulk material will form slope sides in the container along whichcoarser and/or heavier particles will fall down and collect at the footof the slope to a greater degree than finer and/or lighter particleswill do. This separation process occurs if the bulk material has a dryconsistency, such as gravel, but also in the case of moist bulkmaterial, such as wet cement-concrete wherein coarser and/or heavierparticles will drop to the bottom of the container to form slopes in acorresponding way. This will cause a separation inside the containerinto regions containing coarser and/or heavier particles and regionscontaining finer and/or lighter particles, but the sizes of theseregions will depend on variations in composition of the supplied bulkmaterial. During the feeding out of the bulk material from the containerby means of continuously working conveyors, such as worm conveyors,endless conveyor belts, continuously working scraper conveyors, tubeconveyors and the like, the size and/or mass distribution of theparticles will vary in the fed out material. For example, at the feedingout of asphalt mixtures for road surfacing one has observed a separationof the particle shaped material into coarser and finer fraction, in theroad surface, which has led to impaired wear resistance due to the factthat a certain size fraction may be missing while an other one may existin excess although the material which was supplied had a proper particlesize distribution.

BRIEF DISCLOSURE OF THE INVENTION

The overall purpose of the invention is to solve the above mentionedproblem. The invention herein takes advantage of the observation that,when a bulk material is supplied from above to a container fromessentially a point, or possibly along a line, the bulk material will befractioned in the container in the above mentioned way, which means thata supply of bulk material having a certain particle size and/or massdistribution, or a supply of bulk material having a particle size and/ormass distribution varying over time, to a certain degree will befractioned in a predictable way according to its particle size and/ormass distribution. This observation is taken advantage of according tothe invention in that the feeding out conveyor is dimensioned anddesigned such that it will feed out the bulk material with a volume pertime unit and length unit of the feeding out conveyor adapted to thefractioning which did occur in the container, with the result that thefed out bulk material will achieve a particle size distributioncorresponding to that of the bulk material which was fed in, and thesame time as the particle size distribution will be homogenized,utilizing the previous fractioning in the container known by experience.

More particularly, the above is achieved according to the invention inthat the feeding out conveyor is designed such that the volume of bulkmaterial which is fed out per length unit of the conveyor increasesalong the length of the conveyor from the rear end wall to the front endwall, wherein the increase of the volume of bulk material which is fedout per said length unit in the direction of transportation isproportional to the surface within the corresponding length unit of theupper surface of the bed of bulk material between the end walls and theside walls at equilibrium when the feeding in of bulk material into theconveyor equals the feeding out of bulk material.

Further characteristic features, aspects and advantages of the inventionwill be apparent from the following description of a preferredembodiment, and from the appending claims.

BRIEF DESCRIPTION OF DRAWINGS

In the following description of a preferred embodiment will be made inreference to the accompanying drawings, in which

FIG. 1 schematically shows a container having a feeding out conveyor,where the principles of the invention can be implemented;

FIG. 2 shows the upper surface of the bulk material placed in a systemof coordinates;

FIG. 3 shows a conceivable embodiment of a feeding out conveyor;

FIG. 4 is a side view of a an asphalt surfacing machine to which thereis connected an apparatus according to a preferred embodiment of theinvention;

FIG. 5 shows a view along the line V--V in FIG. 4; and

FIG. 6 is a view VI--VI in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

With reference first to FIG. 1, a container is generally designated 20.It consists of a front end wall 21, which is vertical, a rear end wall22, which also is vertical, and two side walls 23, 24, which have anupper vertical portion and which therebeneath slope downwards/inwards inthe region of a bottom portion 25.

In the bottom portion 25 there is a feeding out conveyor 30, which inthe preferred embodiment consists of a worm screw conveyor, which alongpart of its length extends between the two vertical end walls 21, 22. Inthat area, the feeding out screw 30 is exposed to the bulk materialaccomodated in container 20. The feeding out screw 30 is extended beyondthe container 20 but that part of the screw is designed as aconventional worm conveyor 40.

Bulk material is supplied to container 20 from above by means of afeeding in conveyor which symbolically is indicated by arrow 10.According to the invention, the bulk material is supplied by means ofthe feeding in conveyor 10 adjacent the front end wall 21. The bulkmaterial has varying particle sizes and/or particles densities, whichbring about that the larger and/or heavier particles roll down along thesides of the pile 35 of bulk material which successively is formed inthe container 20. The coarsest and/or heaviest material thereforecollects at the foot 36 of the slope near the rear end wall 22 while thefinest and/or lightest material to a major degree stays on top 37. Atequilibrium the material is fed out by means of the worm screw conveyor30 at the same rate as it is being supplied by the feeding in conveyor10.

The purpose with the apparatus is that the bulk material which is fedout from the container 20 shall have a substantially improved homogenityas compared to that which it has in container 20 and preferably even abetter homogeneity than it had when it was supplied by means of thefeeding in conveyor 10. According to the invention, the material isallowed to roll or slide down as above described to cause a separation.Since the side walls slope inwards, the distance between these walls inthe region of the upper surface of the bed of bulk material willcontinuously be smaller from the front end wall to the rear end wallbecause of the sloping pile shape, such that the upper surface of thebed of bulk material will get the shape of a wedge when viewed fromabove. Since the material rolls or slides down also towards the sidewalls, the slope will get a rounded shape, and if the upper surface ofthe bed of bulk material was flattened out it would get the shape shownby the dashed line in FIG. 1. This line can be approximized with astraight line wherein a surface according to the continuous line in FIG.2 is achieved.

Without binding the invention to the theories which shall be explainedin the following, it is the opinion of the applicant that there is arelationship between the upper surface of the bed of bulk material andthe volume of the worm conveyor, in that the increase of fed out volumeof bulk material ΔV_(n) per length unit n in the direction oftransportation of the worm is proportional to the surface A_(n) of theupper surface A of the pile 35 of bulk material between the end wallsand the side walls within the corresponding length unit n, wherein V_(n)corresponds to the volume in the worm within the length unit n. Thusthere is a functional relationship V_(n) =f(l), where l is the extensionof the feeding out conveyor 30 in the direction of transportation. Theupper surface of the bulk material can be calculated according to thefollowing:

A system of coordinates is applied to the surface according to FIG. 2,wherein the line along one of the long sides of the container can beexpressed as: y=kx+b, where k is the inclination of the line, i.e. tany/x, and b is half the breadth of the line of contact of the surface tothe rear end wall 22.

The area A above the x-axis can be expressed as:

    ∫(kx+b)dx and over the entire surface 2∫(kx+b)dx=[kx.sup.2 +2bx]

Since the increase of volume within any length unit according to theabove discussion is proportional to the upper surface for the samelength unit it is thus derived that ΔV_(n) =Ka_(n) and thus that

    ΔV.sub.n =K[kx.sup.2 +2bx].sub.x.sup.x+n

where K is a constant which can be calculated empirically and depends onthe dimensions of the container, its design, fill degree and type ofbulk material, which factors also the constants k and b are dependentof. The expression therefore can be simplified to

    ΔV.sub.n =[K.sub.1 x.sup.2 +K.sub.2 x].sub.x.sup.x+n

From a practical point of view one can, according to the above, statethat the increase of worm volume is an exponential function--a concavefunction--of the extension of the worm conveyor in the direction oftransportation for a container defining an upper surface as above.Further it is known that the slide angle of the bulk material lies inthe range 35°±5° and with a known design of the container a goodstarting value for the empirical calculations can be made. The wormvolume of the conveyor worm thus can be calculated by increasing theworm volume from a start value according to the expression V_(n)=V_(n+1) +ΔV_(n).

Due to the fact that the worm is designed such that the worm volumeincreases towards the feeding out opening according to the expressionabove, an equalisation of the material and a homogenisation of thefractions will occur, i.e. the material in the bed will move essentiallyvertically downwards in the bed. If the worm were not designed inaccordance to the above description, for example if it had a constantworm volume along its entire length, there would, according to the abovetheories, and according to experiments carried out in practice, be takenout more material at the rear end wall wherein the slope would besteeper and a more and more increased slide would occur, with the resultthat predominantly coarser fractions would be fed out.

According to the above discussion, in order to achieve an equalisationof the fed out material, the worm increase would be represented by aconvex function, when the feeding in conveyor instead would feed in thebulk material adjacent to the rear end wall 22, i.e. the worm volumeincrease would be reduced in the transport direction according 30 to theinverse to the above given function for ΔV_(n).

Presuming that the walls of the container are not inclined inwards, butare substantially parallel, the top surface of the bulk material isessentially rectangular, and according to the above discussion thefeeding out worm thus should be designed to have a linear increase ofthe worm volume. An embodiment of that type could be conceived if thebottom surface of the container was provided with a number of parallelfeeding out worms which covered the major part of the container bottom.

The above can be achieved with various designs of the transport worm 30;wherein that the outer diameter D of the worm is constant, while thediameter d of the worm core is getting smaller in the transportdirection of the worm; wherein that the outer diameter of the worm isincreased in the transport direction of the worm while the diameter d ofthe worm core is constant; and/or wherein that the outer diameter D ofthe worm and the outer diameter d of the worm core are constant, whilethe increase of the fed out volume of bulk material per length unit Δlis achieved through a corresponding increase of the pitch angle α of theconveyor worm (conveyor screw) in the direction of transportation.

A preferred embodiment of the conveyor worm 30 is shown in FIG. 3.According to this figure the outer diameter D and the pitch angle α ofthe worm is constant, while the outer diameter d of the worm (screw)core gets smaller. An approximation of the ideal worm increase has beenmade by constructing the worm (screw) core by means of sections 30^(I-V)which either are cylindrical or conical with different, successivelyincreasing conicity in the direction towards that end of the containerwhere the bulk material is supplied, i.e. according to the embodiment inthe direction towards the feeding out end. The shown conveyor wormconsists of five sections, wherein the last section 30^(V) is locatedoutside the container and intended to transport the bulk materialfurther on. This provision facilitates the manufacturing of the screw,such that it adopts a shape which approximately corresponds to the idealone, wherein a sufficient function is achieved.

In that embodiment of the apparatus for homogenizing bulk materialaccording to the invention which is shown in FIGS. 4-6, it consists of amobile asphalt surfacing machine 1. Details in the apparatus which havecorrespondence in FIG. 1 have been given the same reference numerals asin FIG. 1.

The apparatus includes a primary container 5 provided with a transversefeed worm 6; a feeding in conveyor 10; a container 20; two parallelfeeding out worms 30 and 30' which via extensions 40, 40' are connectedto a transverse distribution worm (not shown) for applying asphaltmaterial on a road surface. The primary container 5 is a containerwhich, when full, accommodates about 1 ton of asphalt material, providedfor filling from a lorry platform 2. The transverse feed worm 6 has anincreasing worm volume in the direction of transportation as well as thein feeder 10 which likewise has an increasing worm volume in thedirection of transportation in that part which is located in the regionof the primary container 5 in accordance with the principles of theinvention described in the foregoing. The feeding in worm 10 has itsoutlet opening 15 at the substantially vertical end wall 21 of container20, which is located foremost forwards as seen in the direction oftransportation of the feeding out worms 30 and 30'. The container 20,which has a volume of about 2.5 m³, has two substantially vertical,opposite end walls 21, 22 perpendicularly to the feeding out directionof the feeding out worms 30 and 30', and to longitudinal side walls 23,24 which partly slope inwards. The rear end wall 22 is substantiallylonger than the front end wall 21, and the upper edge of the side walls23, 24 slope from the front end wall 21 rearwards towards the rear wall22. The container has in its bottom portion a rectangular horizontalsection which decreases downwards, said section having a constant lengthin the feeding out direction of the bulk material. The feeding out worms30 and 30' are congruently dimensioned, but one of them is left handthread and the other one is right hand thread and they have oppositedirections of rotation. They are provided to rotate in a feeding outchamber 38 beneath the bottom portion by means of driving means whichare not shown.

The mobile asphalt surfacing machine 1 is moved on a road surface duringasphalt surfacing. The primary container 5 is filled from a lorry infront of the asphalt surfacing machine, wherein the lorry discharges theasphalt material from its platform 2 down into the primary container 5.The primary container 5 also serves as an intermediate store when alorry has been emptied and before a new one has been connected to themachine. The asphalt material is supplied to container 20 from theprimary container 5 by means of the transverse conveyor worm 6 and thein-feeder conveyor 10, both of them being dimensioned with increasingworm volume in order to smooth out the fractioning of the bulk materialon the lorry platform 2 and in the primary container 5 in accordancewith the principles of the invention explained in the foregoing. Thefeeding in conveyor 10 extends from the bottom portion of the primarycontainer parallel with the upper edges of the side walls of thesubsequent container 20 and the material is delivered in the container20 adjacent the front wall 21 continuously and substantially at the samerate as the asphalt material is fed out from the container 20 by meansof the feeding out worms 30 and 30'. The extensions of the feeding outworms 30 and 30' are conventional worm conveyors 40 and 40' which feedthe asphalt material forwards to a transverse distribution worm which isnot shown but which has to the object to distribute the asphalt materialover the breadth of that part of the road surface which shall besurfaced.

The asphalt material in container 20 has a level such that the entirefeeding out worms 30 and 30' are covered with asphalt material. Atequilibrium between supplying asphalt to and discharging asphalt fromcontainer 20, the asphalt material will form a pile having differentparticle size and/or particle mass distribution in different parts ofthe pile. Through the design of the feeding out worms 30 and 30'according to the invention, the feeding out worms 30 and 30' bring awaya predetermined volume per length unit and time unit in proportion tothe surface area of the bed of bulk material lying above the length unitin the container 20. The bulk material within each vertical volumesegment of the bed of bulk material, such as the volume segment Vs_(n)lying under the surface An, will successively sink essentiallyvertically down towards the feeding out conveyor. This can be expressedsuch that the bulk material within each part of the bed of bulk materialin the container, through the method and the apparatus according to theinvention, will sink from the upper surface essentially verticaldownwards towards the feeding out conveyor, which feeds out the materialfrom each segment at the same rate as new material is supplied to theabove lying surface segment of the surface of the bed.

It should be understood from the above description that the worm volumeincrease is not necessarily achieved by one worm (screw). What isimportant is that the bulk material in all parts of the container isfed/sinks essentially vertically downwards because of the desiredincrease of volume of the worm. How this is achieved, by one or severalworms, or by other types of feeding out means, is not essential.

A number of advantages are achieved by the apparatus of the invention.Rather than necessarily avoiding the separation into fractions, whichalmost always occurs when feeding out bulk material at any spot, thefact that separation occurs is utilized for the achievement of a goodequalization and homogenizing of the bulk material. The apparatus andits principles can be utilized for in principle all sorts of bulkmaterial, such as sand materials, gravel, stones, asphalt-concrete, andthe like. It shall therefore be understood that the invention is notrestricted to the embodiment described above and shown in the drawingsbut can be modified within the frame of the appending claims.

We claim:
 1. In a method for homogenizing particulate bulk materialcomprising: feeding in the bulk material into a container (20) having arear end wall (22), a front end wall (21) and two side walls (23, 24),said feeding in being from above and close to one of said end walls sothat the bulk material is caused to slide downwardly towards an other ofsaid end walls such that a bed of bulk material is formed in thecontainer that has a sloping profile and heavier and/or coarser factionsof the bulk material slides further along the profile than lighterfractions; and feeding out the bulk material from a bottom portion ofthe container by means of a conveyor (30, 30') which has a lengthextending from at least the rear end wall to the front end wall andbeing exposed to the bulk material in the container so that the feedingout is in a feeding direction towards the front end wall;the improvementwherein the feeding out is performed by a feeding out conveyor havingmeans for conveying volumes of bulk material per length unit (n) of theconveyor which increase in the feeding direction proportionally todiscrete surfaces (A_(n)) of the upper surface (A) corresponding tolength units (n) of the feeding out conveyor vertically thereunder sothat the bulk material is caused to move essentially downwardly in thebed of bulk material when the feeding in and feeding out of the bulkmaterial is at equilibrium.
 2. Method according to claim 1, wherein thebulk material is fed out from the container in a substantiallyhorizontal direction.
 3. Method according to claim 1, wherein the bulkmaterial is fed out in the same end of the container as where the bulkmaterial is supplied.
 4. In an apparatus for homogenizing particulatebulk material, comprising a container (20) having: a rear end wall (22);a front end wall (21) and two side walls (23, 24); a feeding in conveyor(10) for feeding in the bulk material from above and into the containerto provide in the container a bed of the bulk material having an uppersurface (A) and which bed of bulk material extends from the rear wall tothe front wall; and a feeding out conveyor (30, 30') at a lower part ofthe container which has a length extending at least from the rear endwall to the front end wall and being exposed to the bulk material in thecontainer so as to provide a feeding out of the bulk material in afeeding direction towards the front end wall;the improvement wherein thefeeding out conveyor (30, 30') has means for conveying volumes of thebulk material per length unit (n) of the conveyor which increase in thefeeding direction proportionally to discrete surfaces (A_(n)) of uppersurface (A) corresponding to length units (n) of the feeding outconveyor vertically thereunder when the feeding in and feeding out ofthe bulk material is at equilibrium.
 5. Apparatus according to claim 4,wherein at least a portion of the side walls slope inwards towards thefeeding out conveyor (30, 30'), a feeding conveyor (10) is provided tofeed in the bulk material into one end of the container adjacent to oneof the end walls, and wherein the volume of bulk material (ΔV_(n)) perlength unit increases non-lineary in the feeding direction.
 6. Apparatusaccording to claim 4, wherein the volume of bulk material increasesaccording to an essentially exponential function in the feeding when thefeeding in conveyor feeds in the bulk material adjacent to the front endwall and decreases essentially according to an inverse of theexponential function when the feeding in conveyor feeds in bulk materialadjacent to the rear end wall.
 7. Apparatus according to claim 4,wherein the increase of volume of bulk material (ΔV_(n)) within anylength unit (n) is proportional to the surface (A_(n)) within thecorresponding length unit of the upper surface of the bulk material withconstants (K₁, K₂) according to the formula ΔV_(n) =(K₁, K₂)A_(n) =[K₁x² +K₂ x]_(x) ^(x+n), which constants are empirical units.
 8. Apparatusaccording to claim 7, wherein the volume of bulk material (V_(n)) forany length unit (n) along the length (l) of the feeding out conveyor, iscalculated according to V_(n) =V_(n-1) +ΔV_(n), starting from an initialvalue where l=0.
 9. Apparatus according to claim 4, wherein the feedingout conveyor belongs to any of the types of continuously workingconveyors which include worm conveyors, tube feeders, endless conveyorbelts, continuously working scraper conveyors, and bucket conveyors. 10.Apparatus according to claim 9, wherein the feeding out conveyorcomprises two or more worm conveyors working in parallel.
 11. Apparatusaccording to claim 9, wherein worm volume(s) of the worm conveyorbetween the end walls of the container change in relation to a volume ofbulk material in the container in such a way that, at equilibrium, whena supply of bulk material from the feeding in conveyor to the containeris essentially the same as the feeding out of bulk material by means ofthe worm conveyor from the container, the worm volume increases in thefeeding direction over any length unit (n) of the feeding out wormconveyor, in a degree which corresponds to said increase (ΔV_(n)) of fedout volume of bulk material (ΔV_(n)) per length unit.
 12. Apparatusaccording to claim 9, wherein increased worm volume(s) of the wormconveyor is achieved by an outer diameter (D) of the worm conveyor beingconstant, while a diameter (d) of a worm core decrease in the feedingdirection.
 13. Apparatus according to claim 12, wherein the worm coreconsists of sections which are cylindrical and/or conical with differentconicity to provide increase of volume (ΔV_(n)) for each length unit(n).
 14. Apparatus according to claim 9, wherein increased wormvolume(s) of the worm (the transportation worms) conveyor is achieved byan outer diameter (D) of the worm conveyor increasing in the feedingdirection of the worm conveyor, while a diameter (d) of a worm core isconstant.
 15. Apparatus according to claim 9, wherein an outer diameter(D) of the worm conveyor and an outer diameter (d) of a worm core areconstant, while the increase of the volume of bulk material per lengthunit (Δ1) is achieved through an increase of a pitch angle of the wormconveyor (α) in the feeding direction.
 16. Apparatus according to claim4, wherein the end walls are vertical.
 17. Apparatus according to any ofclaims 4, wherein an end wall adjacent to which the feeding in conveyoris higher than an opposite end wall.
 18. Apparatus according to any ofclaims 4, wherein the apparatus also comprises a primary container (5)for bulk material, from which the feeding in conveyor collects the bulkmaterial which it supplies to said container.
 19. Apparatus according toclaim 18, wherein the primary container (5) has a smaller volume thanthe said container (20).
 20. Apparatus according to claim 18, whereinthe feeding in conveyor is provided to feed the bulk material from alower level out of the primary container (5) to a higher level above thesubsequent container (20).
 21. Apparatus according to claim 18, whereinthe apparatus comprises the following units arranged after oneanother inseries: the primary container, the feeding in conveyor, the saidcontainer, and the feeding out conveyor.
 22. Apparatus according toclaim 21, wherein the apparatus is connected to a asphalt surfacingmachine.
 23. Apparatus according to claim 22, wherein the feeding outconveyor extends beyond the front end wall all the way to said asphaltsurfacing machine.